This invention relates to non-stick coating systems that can be applied by roller coating to smooth substrates. More particularly, it relates to two-layer and at least three-layer coating systems which permit deep drawing of the coated substrates.
Generally in the art a metal substrate is roughened by some means before the first layer of coating is applied so that mechanical bonding will assist chemical adhesive means in holding the coating onto the substrate. Typical roughening means include acid etching, sand-blasting, grit-blasting, and baking a rough layer of glass, ceramic or enamel frit onto the substrate. The problem of adhesion of non-stick coatings to substrates is exacerbated by the nature of the coatings. If the coating is optimized for release to prevent food particles from sticking to it, for easy clean-up after cooking or durability, or to facilitate low friction sliding contact, almost by definition there will be difficulties in making it adhere well to the substrate.
The substrate can be metal, often aluminum of stainless steel used for cookware or industrial applications, or it could be used for an industrial article such as a saw made of carbon steel. Whatever the substrate or the application, if it is necessary to roughen the substrate to make the coating adhere, that at least adds cost and can cause other difficulties including creating a rough profile which can protrude through the coating or cause a roughening of the surface both referred to as telegraphing. This is especially undesirable when smoothness is sought, such as for saws and steam irons. The environmental cost of disposing of etchant materials can be significant.
Efforts in the past to provide non-stick roller coatings for smooth substrates include two PCT patent publications of 25 Jun. 1992, WO92/10309 on "Non-stick Coating System With PTFE and PFA or FEP For Concentration Gradient" and WO92/10549 on "Non-Stick Coating System With PTFE Of Different Melt Viscosities For Concentration Gradient," both in the name of H. P. Tannenbaum. Perfluoropolymers such as polytetrafluoroethylene (PTFE) of two different melt viscosities, or PTFE plus a copolymer of tetrafluoroethylene with hexafluoropropylene (FEP) or with perfluoroalkylvinylether (PFA), are used with a binder of polyamide imide (PAI) or polyether sulfone (PES). U.S. Pat. No. 5,230,961--Tannenbaum (Jul. 27, 1993) teaches the use of PTFE and FEP with PAI and/or PES to give concentration gradients, but without teaching the combination of PAI and polyphenylene sulfone (PPS) needed to give the level of post formability with roller coated needed for some applications.
Post-formable fluororesin-coated articles are taught in U.S. Pat. No. 5,106,682--Matsushita (Apr. 21, 1992), using a roughened aluminum substrate and a two-layer coating of which the first layer is filled and the second layer is not filled. It would be desirable to have a coating system that can be used on smooth aluminum substrates which are to be substantially formed (post-formed) by deep drawing after coating.
Even more challenging is to provide such a coating system that can be applied to smooth substrates by roller coating and still stand up to deep drawing.
Optimum results for roller coating require further developments.